Geology of the Bridger Range, Montana

Nomenclature and correlation of lithologic subdivisions of the Jefferson and Three Forks Formations of southern Montana and northern Wyoming

“…6) sketched the excellent exposures on the north side of the Gallatin River near the present town of Logan, Mont., where Sloss andLaird (1946,1947) later established the type section. Sloss andLaird (1947, p. 1410) suggested a higher content between the Jefferson and Three Forks Formations than Peale had apparently described, but Sandberg and Hammond (1958, p. 2314-2315 and most other workers, for example, McMannis (1955McMannis ( , p. 13961962, p. 5) and Robinson (1963, p. 28), have reverted to Peale's original contact. A detailed stratigraphic section of the type Jefferson Formation in the S%SE% sec.…”

“…These members appear to be unrecognizable even short distances from the type section (McMannis, 1955(McMannis, , p. 1397Robinson, 1963, p. 28), and the terminology is therefore not in common use. Sandberg and Hammond (1958) recognized that the type Jefferson comprised exact equivalents of the Duperow arid Birdbear Formations of the Williston basin area and designated the Jefferson a group in the subsurface east of the lllth meridian in Montana.…”

“…Sandberg and Hammond (1958) recognized that the type Jefferson comprised exact equivalents of the Duperow arid Birdbear Formations of the Williston basin area and designated the Jefferson a group in the subsurface east of the lllth meridian in Montana. Since then, the widespread distribution of the thick Duperow equivalent and thin Birdbear equivalent in the outcropping Jefferson Formation have been widely recognized (McMannis, 1962;Sandberg, 1963). To forestall the eventual use of these commonly unmappable equivalents as formations conflicting with the long-established Jefferson Formation, a nomenclature based on lithologic subdivisions of the Jefferson is proposed.…”

“…Fossil collections reported by Robinson (1963) supported a dual age for the Sappington, and recent collections by the author suggest that only the uppermost beds are Mississippian. Recent stratigraphic workers, including Christopher (1961) and Ran (1962), also retained the Sappington in the Three Forks, but others, including Achauer (1959), Gutschick, Suttner, and Switek (1962), and McMannis (1955, continued to consider the Sappington as a separate formation. Despite his use of Sappington Formation, McMannis (1955) did not attempt to map it separately in the Bridger Range, 20 miles east of Logan.…”

“…This subsurface nomenclature, however, is not directly applicable to equivalent outcropping rocks because of different concepts of subsurface and surface mappability. A similar uniform surface nomenclature is here proposed to replace various informal subdivisions, such as those used by Haynes (1916), McMannis (1962), Kobinson (1963), and Sandberg (1962Sandberg ( ,1963, and also to replace one subsurface member, which was proposed by Sandberg and Hammond (1958).…”